a) Field of the Invention
The invention relates to a process for producing color picture points of a television picture which are illuminated with light signals of different wavelengths and whose hues are fixed by means of color value signals in a first base system which may be represented in a CIE diagram by corner points of a hue region which are defined via wavelengths of screen phosphors, and a second base system is set up in which the wavelengths of the light signals differ from those of the first base system and which shares a common range of hues with the first base system, and light signals are generated from at least three signals which are the color value signals or contain the latter by carrying out a transformation from the first base system to the second base system via a matrix. The invention is further directed to a television projection system for producing color picture points of a television picture with at least three light sources which may be controlled in intensity for light signals of different wavelengths, the hue of each picture point being fixed by color value signals in a first base system which may be represented in a CIE diagram by corner points of a hue region which are defined via wavelengths of screen phosphors, and the wavelengths of the light signals determine a second base system which differs from the first base system and has a range of hues in common with the first base system, with an input circuit for generating at least three electrical signals which are proportional to the color value signals or contain a mixture thereof, and further with a control device for controlling the light sources, which control device is triggered by the signals of the input circuit and has a circuit which can transform every hue from the first base system into the second base system via a matrix, in particular for carrying out the process mentioned above.
b) Background Art
A television projection system using such a process was described in the "Proceedings of 6th International Quantum Electronics Conference" 1970 by Yahiko Yamada, Manabu Yamamoto, and Sadao Nomura in an article entitled "Large Screen Laser Color TV Projector". Three differently colored lasers are used as light sources and are modulated by means of DKDP crystals and combined to form a common beam of light by means of a system of dichroic mirrors. This beam of light is projected on to a screen via a deflection device. The deflection device raster scans the beam of light in accordance with the picture frequency and line frequency in such a way that a color television picture is imaged on a screen.
However, this type of projected picture differs substantially from color pictures known from the television tube of a conventional color television receiver, since color television tubes produce colors by means of screen phosphors which emit a relatively broad color spectrum, whereas lasers are sources of monochromatic light. For color-correct reproduction in conventional televisions, the color sensitivity of television cameras is coordinated to the screen phosphors. In projections with a laser system, however, this leads to significant color distortions. Another cause of substantial color distortions is due to the lack of availability of laser light sources for all desirable wavelengths, so that not all of the desired hues can be produced even by a careful selection of lasers.
The article by Yamada et al. mentioned above, describes a television projection system in which a video picture is produced by means of three lasers with wavelengths 488 nm, 514 nm and 647 nm. Thus, the colors of the lasers do not conform to the colors of the NTSC video signal. Therefore, the authors of the article mentioned above suggest that the hues which can be achieved by the lasers be corrected for the hues transmitted via the NTSC standard by means of a matrix circuit. However, it is expressly noted that this only enables a restricted correction of hue.
In the book "Farbmetrik und Farbfernsehen [Chromatometry and Color Television]" by H. Lang, R. Oldenbourg Verlag, Munich/Vienna, 1978, it is stated that hues may be transformed from one base system to another base system by matrix operations. The stated preconditions enabling such transformation are essentially the existence of base systems and components for hues and the ability to represent them as vectors in coordinates x, y and z. However, it is indicated that difficulties may arise with spectral colors in which some components in the vector space are negative. Negative color components cannot be physically realized. Therefore, when negative components occur, the hue vector itself is not transformed, but rather another hue vector which is supplemented by an additional positive vector is transformed.
With the further development of television projection systems, the hue transformation was also no longer used. Thus, Teiichi Taneda et al. in "High Quality Laser Color Television Display" NHK Laboratories Note (1972), serial number 152, describe a laser projection system using wavelengths of 477 nm, 515 nm and 633 nm which, with the exception of green, are located in the vicinity of the colors according to the NTSC standard. Despite deviations of the produced hues from the NTSC colors, a color transformation is dispensed with.
Sometime later (1973), the authors describe this system again in "Journal of the SMPTE" vol. 82 page 470, without any discernable evidence of progress in the color matching.
Color transformations are also not used in later developments of large picture projection devices as can be seen, e.g. from WO A-88/01823, from "Development of a Large Screen High Definition Laser Video Projection System", Tony Clinic, SPIE, vol. 1456, "Large-Screen-Projection" Avionic and Helmet Mounted Displays, 1991, pages 51 to 57, and from "Laser-Based Display Technology Development at the Navel Ocean Systems Center (NOSC)" Thomas Phillips et al , SPIE, vol. 1454, Beam Deflection and Scanning Technologies, 1991, pages 290 to 298. Rather, the course pursued in development was to select the color of the light sources in such a way that they reproduce the colors of the screen phosphors as accurately as possible.
In order to match the laser colors to the colors of the screen phosphors as accurately as possible, dye lasers can be used, by means of which the wavelengths of the produced laser light can be shifted and their hue matched to those of the screen phosphors. However, dye lasers have only limited efficiency and this technique drastically reduces laser output. To provide a higher output would make the system extremely expensive. Substantially higher outputs would also be impossible to realize at a reasonable cost (see "Fernsehund Kino-Technik" 1974, No. 6, page 169). The toxicity of the laser-active substances presents a further disadvantage of dye lasers in that it creates serious disposal problems, chiefly because the useful life of commercial dye lasers is very limited. Nevertheless, dye lasers have been used for color shifting in laser projection systems (e.g. EP-A-0 084 434).
The selection of laser wavelengths in the article by Tony Clinic mentioned above is also effected by means of dye lasers, a wavelength between 600 and 620 nm is selected for red, a wavelength of 514.5 nm for green, and a wavelength between 457 and 488 nm is selected for blue. The hue is matched to the screen phosphor by means of a circuit in which the control signals for red, green and blue are transformed via a gamma corrector in order to achieve an acceptable range of hues for HDTV. However, it is not entirely clear how a gamma corrector, which is essentially nonlinear, can achieve an improved hue reproduction, so that it must be assumed that the hues achieved by this system also do not reproduce natural colors.
It is asserted in the book "Die Einfuhrung des hochauslosenden Fernsehens [The Introduction of High-resolution Television]" by A. Felsenberg, Verlag Gerhard Spiehs, Kottgeisering, October 1990 (p. 26) that color reproduction for HDTV must be considerably improved. A number of suggestions are examined in this regard, including a suggestion for the use of imaginary primary colors which are restored by calculation to real colors in the receiver. However, this suggestion, i.e. to achieve improved hues by matrix transformations, is also subject to criticism in view of a risk of impaired quality due to interference.